1. Field of the Invention
The present invention relates to a method of compensating profile data capable of machining workpieces such as a cam and the like in high machining efficiency and accuracy. More particularly, the present invention pertains to a numerical controller for practicing the method and to a machine tool provided with a numerical controller for practicing the method.
2. Description of the Related Art
With improvements in performance and precision of various machines, higher machining accuracy has been demanded for components of such various machines. For instance, a camshaft and a crankshaft of an automobile engine are required to have a cam and a crankpin whose sliding surfaces are to be of a high dimensional accuracy and a high degree of a surface roughness.
However, when machining the aforementioned cam (a non-circular workpiece) and the crankpin, especially when grinding the cam and the crankpin, the cam and the crankpin cannot be machined by simply feeding a tool (e.g. a grinding wheel) through an amount to be machined relative to such rotating workpiece. Accordingly, it is necessary to machine the workpiece by controlling the rotational angle of the workpiece and the feed amount or position of the tool in synchronism relation with each other. Generally, the machining process of the workpiece is performed by numerically controlling a work spindle and a tool feed axis in accordance with predetermined profile data (hereinafter, merely referred to as profile data).
However, the above-described type of machining process applied to the workpiece is too complicated to easily machine the workpiece with a high machining accuracy along an ideal shape. For example, some dimensional errors may occur. In order to reduce the occurrence of the dimensional errors, various methods of compensating the profile data have been developed. Some of such methods for compensating the profile data are described hereinbelow taking cam grinding as an example.
In a typical method, an actual machining operation is performed based upon theoretical profile data which is geometrically obtained from ideal cam lift data (i.e. finished shape data). Measured lift data is obtained by actually measuring lift amounts at plural angular positions of the cam by means of a cam measuring device after performing the actual machining operation. Machining errors are then confirmed by comparing the measured lift data with the ideal lift data. Compensated profile data is finally obtained by directly reflecting the machining errors on the ideal lift data or on the theoretical profile data.
Another method of compensating the profile data is disclosed in a Japanese Patent Publication No. 6(1994)-22778. A grinding machine (i.e. a machine tool) is operated in accordance with theoretical profile data with a camshaft (i.e. a workpiece) being set on a work spindle, wherein the operation is performed under a non-load condition, namely without engaging a grinding wheel with the workpiece. During this operation, actual rotational angles of the work spindle and actual feed amounts or positions of a wheel head (i.e. a tool feed axis) are detected by respective encoders so as to obtain effective values thereof. Errors of the effective values are obtained by comparing the effective values with command values according to the theoretical profile data. Compensated profile data is obtained by reflecting the errors on the theoretical profile data.
However, where the machining accuracy is to be further higher, the machine errors cannot be thoroughly compensated for by the use of any one of the above-described methods of compensating the profile data. This problem may be considered for the reasons that there are plural error-causing factors or systems and that error components (error characteristics) caused by the corresponding error-causing factors or systems have been mixed in the entirety of machining errors. That is, even when particular error components can be compensated, other error components of the other systems may still remain, so that the machining errors are difficult to compensate completely. Further, when a particular error component is compensated excessively, a new error may occur.
For instance, let it be the case that error lift data is obtained from the difference between the measured lift data obtained by measuring the machined workpiece and the original ideal lift data and that the ideal lift data which represents the base of shape information of the workpiece is then modified by the error lift data so as to generate new ideal profile data through a lift-profile conversion processing. In this case, the ideal profile data may fluctuate in dependence upon a machining condition of the workpiece from which the measured lift data was obtained. The error components of the control system calculated from the fluctuated ideal profile data may also fluctuate. Therefore, according to the above-described method of compensating the ideal lift data by the use of the error lift data, accurate extraction of the error components caused by the control system may not be performed.
When an attempt is made to compensate the entirety of machine errors through the repetition of a try-and-error effort for adjusting correction levels (weights) for error components of a particular system, a man-power for the compensation may be extremely increased. Further, such compensation procedure is required to be performed every time a workpiece or a tool is replaced with another one, which would lead to a troublesome and inefficient operating process.
Accordingly, it is necessary to provide an improved method of compensating profile data which method is capable of efficiently obtaining compensation profile data for a higher accuracy machining. Further, it is necessary to provide a numerical controller and a machine tool which are capable of practicing the above-described profile data compensating method.
The present invention has been created to solve the aforementioned problems, and it is a primary object of the present invention to provide an improved method of compensating profile data which method is capable of efficiently obtaining compensated profile data for a higher accuracy machining by dividing the entirety of machine errors into error components of the control system due to characteristics of those ranging from a numerical controller up to a work spindle and a tool feed axis and error components of mechanical systems due to characteristics of those mechanical elements including driving mechanisms, tools, other mechanical elements of the machine tool, so that the error components can be made clear separately of the control system as well as of the mechanical system.
In one aspect of the present invention, a method of compensating profile data is provided to be practiced in a numerical controller for a machine tool which machines a workpiece by numerically controlling a work spindle and a tool feed axis in accordance with profile data composed of rotational angle commands of the work spindle for rotating the workpiece as an object to be machined and feed commands of the tool feed axis for displacing a tool for machining the workpiece in synchronism with the rotation angle of the work spindle. The method comprises an effective profile data generating step of generating effective profile data from effective data which is obtained by numerically controlling the work spindle and the tool feed axis in accordance with theoretical profile data obtained geometrically based upon finished shape data of the workpiece under the condition that the tool feed axis is off-set by a machining amount (i.e., allowance) or more relative to the workpiece so as not to actually machine the workpiece being set on the work spindle so that the machine tool is operated without having machining load applied thereto, and by actually detecting the rotation angles of the work spindle and the feed amounts or positions of the tool feed axis in the state of such non-machining load acting on the machine tool; a first error profile data generating step of generating first error profile data corresponding to the difference between the effective profile data and the theoretical profile data; a first compensated profile data generating step of generating first compensated profile data by compensating the theoretical profile data for the first error profile data; a measured profile data generating step of generating measured profile data through a lift-profile conversion processing based upon measured lift data which is obtained by numerically controlling the work spindle and the tool feed axis in accordance with the first compensated profile data to actually machine a workpiece and by measuring the workpiece actually machined; a second error profile data generating step of generating second error profile data corresponding to the difference between the measured profile data and the first compensated profile data; and a second compensated profile data generating step of generating second compensated profile data by compensating the first compensated profile data for the second error profile data or by compensating the theoretical profile data for the first error profile data and the second error profile data.
More specifically, the error components of the control system are extracted as the first error profile data and the error components of the mechanical system are extracted as the second error profile data. The first error profile data corresponds to the difference between the effective profile data and the theoretical profile data. The effective profile data is generated based upon the effective data which is detected when the machine tool is operated under the non-load condition in accordance with the theoretical profile data. Therefore, the first error profile data does not involve the error components of the mechanical system and is inherent to the control system. The workpiece machined in accordance with the first compensated profile data which is obtained by compensating the theoretical profile data for the first error profile data substantially does not involve the error components of the control system.
The second error profile data corresponds to the difference between the measured profile data and the first compensated profile data from which the error components of the control system has been substantially excluded. The measured profile data is generated based upon the measured lift data obtained by measuring the workpiece which was actually machined in accordance with the first compensated profile data. Therefore, the second error profile data does not involve the error components of the control system and is inherent to the mechanical system. The workpiece machined in accordance with the second compensated profile data which is obtained by compensating for the second error profile data does not involves not only the error components of the mechanical system but also the error components of the control system having been excluded on a previous stage.
Further, in the method according to the present invention, because error components which cause the machining errors are separated into those for the control system and those for the mechanical system and because the errors involved in one of the systems is compensated independently of those involved in the other system, the profile compensation can be performed efficiently. Specifically, the error components of the control system does no vary essentially. Therefore, where the driving system is deteriorated with age, where the workpiece is exchanged with another one, or where the tool is exchanged with another one, it is sufficient to consider varying the error components of the mechanical system only. In this case, the profile data compensating operation can be efficiently performed by regenerating the second error profile data and by updating the second compensated profile data.